This is the 7th most frequent Croatian word.
koji
"Koji" means "which," "who," or "that" in English, depending on context.
Here, 'koji' functions as a relative pronoun referring to the noun 'projekt', connecting it to the clause 'smo završili jučer'.
Ovo je projekt koji smo završili jučer.
This is the project that we finished yesterday.
In this case, 'koji' acts as a relative pronoun describing the noun 'pas', and it introduces a relative clause.
Vidjela sam psa koji je izgledao izgubljeno.
I saw a dog that looked lost.
Here, 'koji' again is a relative pronoun, referring to 'muškarac', specifying which man.